Mesoscale Precipitation Discussion 0707
NWS Weather Prediction Center College Park MD
220 PM EDT Tue Aug 21 2018

Areas affected...MD INTO CENTRAL AND NORTHERN VA...SOUTHERN DE

Concerning...Heavy rainfall...Flash flooding possible

Valid 211820Z - 212320Z

Summary...Slow moving storms tracking through a high moisture
environment along a frontal boundary could produce a flash flood
threat into the early evening.

Discussion...Storms forming along a frontal boundary extending
from central PA into northern MD into the southern Delmarva
Peninsula are forming on an axis of 1500-2000 J/KG MLCAPE. This is
reflected in the GOES-16 visible images, where topping are highest
along this axis. The most recent RAP analysis showed precipitable
water values between 2.00/2.25 inches, which is supporting hourly
rainfall rates between 2.00 and 2.50 inches across portions of
northern VA into nearby central and southern MD.

As the instability peaks during the late afternoon, the convection
is expected to expand in coverage as the front moves slowly north.
Increasing divergence ahead of a mid level trough over the Ohio
Valley could aid in coverage as well. At this point, based on
cloud cover near and north of the front, instability may end up
being the limiting factor for a more expansive flash flood threat.
Having said that, given the relatively slow southwest motion to
the storms in the deep tropical like moisture, hourly rainfall
rates over 2.00 inches can be expected across this area.

Much of the high resolution guidance has not captured this
activity well, with the 12z regional GEM having the best handle on
the convection. Given what is occurring now, flash flooding is
considered possible, since the extent of the coverage is unclear.
However, where convection does develop and track, there is a
threat for local 4.00 inch rainfall amounts through 21/23z.

Hayes

ATTN...WFO...AKQ...CTP...LWX...PHI...

ATTN...RFC...MARFC...

LAT...LON   39727702 38767555 38087476 37347489 37077557
            37347623 38517824 39097858 39577822